ImageHeight and ImageWidth duplicates not the same?

[Beholder3]

Loook ed at a Pentax K-5 DNG file and just realised this:

---- ExifTool ----
ExifTool Version Number         : 8.95
---- File ----
File Name                       : IMGP3854.DNG
---- EXIF ----

Subfile Type                    : Full-resolution Image
Image Width                     : 4992
Image Height                    : 3284
Subfile Type                    : Reduced-resolution image
Image Width                     : 4928
Image Height                    : 3264

20 pixels in height and 84 pixels in width are for what? Where?
Windows only shows the "reduced-resolution image" dimension as the one dimensions. 

[Phil Harvey]

RAW files typically store information from all sensor pixels, but not all pixels are imaging pixels.  Usually there is a mask which blackens a frame around the image.  My guess is that this mask is 10 pixels on the top and bottom, and 42 pixels each side of the image.

- Phil

[Beholder3]

Ah, yes that could be it. Maybe it also has something to do with the shake reduction needing extra pixels around the edges if it's tilted.

[Phil Harvey]

I don't think it could be for shake reduction -- the entire sensor moves to compensate for shake/rotation.  No pixels are harmed in the process.

- Phil